My son's also 10, and we've had this issue on an off. We've explained that school isn't ALWAYS exciting and interesting. It just isn't. You sometimes have to suck it up and do things that are boring. Slacking off is only going to result in spending MORE time on the boring crap, so get it overwith and put in enough effort to move on to the next thing.

This is assuming your kid is at the appropriate grade level, of course.

~amy